Squirrels and rats are both rodents, but they sit on separate branches of the rodent family tree that split apart roughly 70 to 80 million years ago. That makes them about as distantly related as two rodents can be while still belonging to the same order. To put it in perspective, the entire order Rodentia diverged from primates around 90 million years ago, so squirrels and rats separated from each other not long after rodents became their own group.
Same Order, Different Suborders
The order Rodentia is enormous, containing over 2,050 species across 29 families and 468 genera. It’s the largest order of mammals by species count. Within that order, the traditional classification system (based on skull shape and jaw muscle arrangement) places squirrels and rats in two distinct suborders: Sciuromorpha (squirrel-like rodents) and Myomorpha (mouse-like rodents).
More recent molecular studies, which classify rodents by analyzing DNA rather than physical traits, confirm the separation. Genetic analysis groups rodents into three major lineages: a squirrel-related clade, a mouse-related clade, and a porcupine-related clade. Squirrels cluster with dormice in their lineage, while rats and mice group with beavers and pocket gophers in theirs. When researchers have built phylogenetic trees using mitochondrial DNA, the squirrel family (Sciuridae) and the rat family (Muridae) consistently form two completely distinct branches with no overlap.
What Makes Them Both Rodents
The reason people often lump squirrels and rats together is that they share the defining rodent trait: a pair of continuously growing incisors on both the upper and lower jaw. These front teeth have hard enamel only on the front surface and softer material behind, which creates a self-sharpening chisel edge as the animal gnaws. This dental structure is universal across all rodents and is the single feature that unites the entire order. Beyond the teeth, both squirrels and rats have compact, relatively flexible bodies, clawed digits, and similar digestive systems adapted for processing plant material.
Key Biological Differences
Despite their shared rodent toolkit, squirrels and rats have evolved very different body plans and sensory systems suited to different lifestyles.
Vision
One of the starkest differences is in their eyes. Squirrels are overwhelmingly diurnal (active during the day), and their retinas reflect that: between 60% and 90% of the light-detecting cells in a squirrel’s eye are cones, the type responsible for color vision and sharp daytime sight. Squirrels can see in both green and blue wavelengths. Rats, on the other hand, are mostly nocturnal. Over 80% of their retinal cells are rods, which are far more sensitive in low light but provide less color detail. Rats do have some cones, but theirs detect green and ultraviolet light rather than blue. The visual processing centers in the brain differ accordingly: squirrels have a more complex relay station between the eye and the brain, with five distinct layers, compared to three in rats.
Body Structure and Movement
Squirrels are a remarkably diverse family of about 279 species across 51 genera, ranging from tiny chipmunks to large marmots to flying squirrels. Their limb bones vary dramatically by lifestyle. Tree squirrels and gliding squirrels have longer, more slender limb bones optimized for climbing, leaping, and resisting the twisting forces of moving through a three-dimensional canopy. Their upper arm bones tend to have rounder cross-sections, which helps absorb stress from multiple directions. Ground squirrels, by contrast, have thicker, more oval-shaped arm bones suited for digging. Rats generally have a more generalized body plan built for running along flat or gently angled surfaces, though some species are decent climbers.
A Useful Comparison
If you’re trying to calibrate how related squirrels and rats really are, think of it this way: squirrels are about as closely related to rats as cats are to dogs. Cats and dogs are both in the order Carnivora but belong to different suborders that diverged tens of millions of years ago. Similarly, squirrels and rats share an ancient common ancestor and the signature rodent incisors, but their lineages have been evolving independently for the vast majority of mammalian history. They look superficially similar because they’re both small, furry, and abundant, not because they’re close cousins.
The relationship is real but distant. A squirrel is technically closer to a beaver (which falls in the mouse-related molecular clade, surprisingly) than you might expect from appearances, and a rat is closer to a hamster or a vole than it is to any squirrel. Shared traits like gnawing teeth, whiskers, and a taste for seeds are features of being a rodent in general, not signs of a tight family bond between these two groups.